24 
SILUEIA 
[Chap. II. 
that such ancient grits and schists, evidently deposited by water from the 
waste of previous rocks, are succeeded regularly, and without disorder, by 
other strata, differing little from the former in composition, in which, at va- 
rious points of the surface of the globe, and at the same level or horizon, we 
obtain distinct relics of animals of a higher organization. Once introduced 
to these well-defined creatures in the 1 Primordial Zone ' of Barrande, or 
true Silurian base, the explorer meets thenceforward with an abundant 
supply of organic remains in all the formations which were successively 
accumulated. 
The accompanying diagram explains at a glance this generalization, as 
established by an appeal to the structure of every well explored country, 
where the rocks exhibit the several terms in this long primeval series. 
General Order of the Primeval Stratified Rocks. 
x 1 2 3456 78 
The strata marked 1 are the Laurentian or oldest known deposits, which 
have been formed out of some preexisting rocks, and contain the Eozoon. 
In No. 2 (the 1 Cambrian' of the British Geological Surveyors), feeble in- 
dications only of former life have been detected. The overlying accumu- 
lations, 3, 4, 5, 6, 7, and 8, stand for the principal formations into which 
the Silurian System, with clearly developed and abundant life-groups, is 
divided. The granite, whether intrusive or metamorphic, is x. 
It may here be noted that in no part of England and "Wales have any 
strata been proved to be of more remote age than the Cambrian, No. 2 of 
the above diagram — though in Scotland the still lower or true basement 
rocks exist, as has been already stated. 
Cambrian or Basement Rocks of the Silurian Region, and their equiva- 
lents in Wales. (See Map and its accompanying Section, colour No. 1.) — 
Let us first review the older rocks in the ascending order, as they were 
first studied in the western tract of Shropshire, where the lowest strata 
containing well-defined fossils are seen to repose upon a vast mass of still 
older and little . altered deposits, in which slight traces only of former 
life have of late years been detected. It will afterwards be shown how 
the same succession prevails in the slaty regions of Wales, to which the 
Silurian classification was subsequently applied. 
The northern portion of the original Silurian Eegion, or the south- 
western part of Shropshire, is marked by a group of hills which presents 
to the eye an outline and a vegetation strikingly different from those of the 
surrounding elevations. When examined in relation to the contiguous 
formations, this mountainous mass is found to be composed of the most 
